In order to protect telephone cables from weather, ground water, vermin and other destructive elements, it is customary practice to run the cables through a protective pipe line referred to as "conduit". Normally, the pipe is of a rigid plastic material such as polyvinyl chloride or acrylonitrile butadiene styrene (ABS). The pipe must be of some limited length, normally about 20 feet or less, to facilitate shipping and handling. Accordingly, there must be some means of coupling the pipes.
It is essential that the coupling have good mechanical integrity to resist the various forces to which the conduit is subjected during and after installation. During installation, it becomes necessary to bend the conduit around corners, thus subjecting the couplings to significant bending moments. Once the conduit system is in place, the telephone cable is installed by pulling it through the conduit system from one end to the other. This subjects the conduit system to pull-out forces, forces along the pipe and coupling axes which tend to separate them. Moreover, after the cable is in place, the pipe is subjected to thermal expansion and contraction.
Due to the potentially costly and disabling results which can follow from the failure of burial ducts for telephone cables, and as a result of untoward experience with prior art couplings, telephone companies have developed stringent requirements for coupling performance. Notwithstanding a long and still continuing search, the development of acceptable couplings has proved to be a problem. There is considerable evidence that the solution to the problem is not obvious. The difficulty of finding an appropriate solution is evidenced by the fact that the cemented rigid plastic coupling, notwithstanding its rather serious disadvantages, is the dominant factor in the telephone conduit coupling field today.
One type of cemented coupling is a short cylinder having an inside diameter approximately equal to the outside diameter of the telephone conduit pipe. The ends of two adjoining sections of the pipe are inserted into such cylinder and "cemented" there by an adhesive. Alternatively, a bell-type coupling may be formed integral with one end of each section of pipe, so that the pipe has a "bell" end and a "free" end. The inside diameter of the bell is approximately equal to the outside diameter of the free end, so that the pipe sections may be joined in series with their free ends inserted in the bell ends and "cemented" with the adhesive. This type of coupling technique, which sounds simple enough, involves fundamental drawbacks pertaining to workmanship control, hazards to health, uncleanliness, flammability and delays.
The procedure for forming these joints begins with applying a volatile solvent, for example tetra hydro furan, to the inside of the coupling and to the exterior surfaces of the adjacent pipe sections before they are joined. Part of the solvent is absorbed by the coupling and pipe surfaces, and part of it dries, giving off vapors into the atmosphere. After a waiting period, which should be carefully timed in relation to the absorption and drying, the cement or adhesive is painted onto the pipe ends and coupling. This cement also contains a volatile solvent and gives off solvent vapors. Then, within a limited time which should be carefully controlled because of the rapid drying characteristics of the cement, the pipes and coupling must be assembled. However, the pipes may not be bent around a corner until the cement has cured for a period of time, e.g. 15 to 30 minutes, after assembly. While this delay can frustrate efficient installation, adding to conduit system costs, premature bending will ruin the pipe joint.
Tetra hydro furan and other solvents are considered hazardous to health. They are toxic and can cause unconsciousness. When cementing must be done in a ditch in which the conduit is to be buried, the vapors can accumulate at levels considered dangerous by government authorities. Some of the solvents used in these cements are highly flammable, creating a further hazard.
Use of the cement can be a dirty and disagreeable job. Spillage of cement and inadvertent contact with wet couplings and pipe ends creates a residue on clothing and worker's hands which is most difficult to remove, generally requiring clean up with the above-mentioned hazardous solvent. Also, because the pipe ends and coupling are in a sticky condition from the solvent and cement for some period of time prior to assembly, contamination of the pipe joints with dirt is a danger.
Considering all the disadvantages of the aforementioned method, it seems evident that if a better way of coupling the conduits were obvious, it would have been adopted long ago. Thus, a need remains for telephone conduit pipe couplings and coupling-pipe combinations which can be quickly and easily joined in the field to form pipe assemblies having stable joints, e.g. joints which can accomodate the usual pull-out forces, which are not adversely affected by bending immediately after installation, which do not invole field application of hazardous or messy adhesives and can be assembled without special techniques. The object of the present invention is to meet this longstanding need.